Single extraction

Let us consider a specific example, viz., the extraction of a solution of 4 Og. of n-butyric acid in 100 ml. of water at 15° with 100 ml. of benzene at 15°. The partition coefficient of the acid between benzene and water may be taken as 3 (or J between water and benzene) at 15°. For a single extraction with benzene, we have ... 

Clearly, a single extraction is not reasonable under these conditions. 

In Example 7.14 a single extraction results in an extraction efficiency of only 60%. If a second extraction is carried out, the fraction of solute remaining in the aqueous phase, ( is given by... 

Furthermore, the extent to which we can effect a separation depends on the distribution ratio of each species in the sample. To separate an analyte from its matrix, its distribution ratio must be significantly greater than that for all other components in the matrix. When the analyte s distribution ratio is similar to that of another species, then a separation becomes impossible. For example, let s assume that an analyte. A, and a matrix interferent, I, have distribution ratios of 5 and 0.5, respectively. In an attempt to separate the analyte from its matrix, a simple liquid-liquid extraction is carried out using equal volumes of sample and a suitable extraction solvent. Following the treatment outlined in Chapter 7, it is easy to show that a single extraction removes approximately 83% of the analyte and 33% of the interferent. Although it is possible to remove 99% of A with three extractions, 70% of I is also removed. In fact, there is no practical combination of number of extractions or volume ratio of sample and extracting phases that produce an acceptable separation of the analyte and interferent by a simple liquid-liquid extraction. 

Thus, a single extraction improves the separation of the solutes by a factor of 2.5. As shown in Figure 12.1, a second extraction actually leads to a poorer separation. After combining the two portions of the extracting phase, the concentration ratio decreases to... 

After the amine is removed, a single extraction with ethyl ether is sufficient to provide a 98% pure product ia ca 75% yield. Lithium borohydride is a hygroscopic, white powder that decomposes slowly at its melting poiat, evolving hydrogen. The heat of formation is —190 kJ/mol (—45.4 kcal/mol). 

Purification of the radioactive tracer was modified to include a fractional sublimation before a single extraction—recrystallization cycle to conserve the tracer material. Microgram samples were prepared in melting point capillaries for assay by mass spectroscopic analysis (Table III), made by direct probe injection of the sample into the ion source (18). The probe was heated rapidly to 200°C, and mass spectra were obtained during vaporization of the sample. Tri-, tetra-, and pentachlorodibenzo-p-dioxins vaporized simultaneously with no observed fractionation. 

Another example of the use of small particle silica is in the analysis of theophylline in plasma, as shown in Figure 5 (40). The clean-up procedure is simply a single extraction of the plasma with an organic solvent. This analysis has also been achieved by reverse phase chromatography (41), and this points out the fact that in some separations (e.g. with components of moderate polarity) either the adsorption or reverse phase mode can be used. 

One of the attractive features of SFE with CO2 as the extracting fluid is the ability to directly couple the extraction method with subsequent analytical methods (both chromatographic and spectroscopic). Various modes of on-line analyses have been reported, and include continuous monitoring of the total SFE effluent by MS [6,7], SFE-GC [8-11], SFE-HPLC [12,13], SFE-SFC [14,15] and SFE-TLC [16]. However, interfacing of SFE with other techniques is not without problems. The required purity of the CO2 for extraction depends entirely on the analytical technique used. In the off-line mode SFE takes place as a separate and isolated process to chromatography extracted solutes are trapped or collected, often in a suitable solvent for later injection on to chromatographic instrumentation. Off-line SFE is inherently simpler to perform, since only the extraction parameters need to be understood, and several analyses can be performed on a single extract. Off-line SFE still dominates over on-line determinations of additives-an... 

Despite these shortcomings, common to any chemical extraction procedure, sequential dissolution techniques still furnish more useful information on metal binding, mobility and availability than can be obtained with only a single extractant. 

Rajagopalan et al. [72] described an electron-capture gas chromatographic assay method for the determination of primaquine in blood. The method involves deriva-tization with heptafluorobutyric anhydride to form the diheptafluorobutyramide derivative after a single extraction at alkaline pH. The derivatives are quantitated by electron-capture gas chromatography. Blood levels of primaquine as low as 8 ng/mL can be measured with good precision. 

C02 solubility. For example, the desired branched aldehyde formed upon hydroformylation of vinylnaphthalene is considerably more soluble in scC02 than its linear isomer. Therefore, the ratio of the branched to linear product increases from approximately 90 10 in the condensed phase to up to 98 2 in the C02 phase in a single extraction step under the low density conditions of catalyst separation. 

If D is large, i.e. > 102, a single extraction may effect virtually quantitative transfer of the solute, whereas with smaller values of I) several extractions will be required. The amount of solute remaining in the aqueous phase is readily calculated for any number of extractions with equal volumes of organic solvent from the equation... 

If the value of > is known, equation (4.5) is useful for determining the optimum conditions for quantitative transfer. Suppose, for example, that the complete removal of 0.1 g of iodine from 50 cm3 of an aqueous solution of iodine and sodium chloride is required. Assuming the value of D for carbon tetrachloride/water is 85, then for a single extraction with 25 cm3 of CC14,... 

Assuming that log P should be at least 5 for an essentially quantitative separation by a single extraction (see p. 57), ApH1/2 should be 5, 2.5 and 1.7 respectively for pairs of mono-, di- and trivalent metals. Selectivity by pH control is greatest, therefore, for trivalent metals and least for monovalent. This is reflected in the slopes of the curves which are determined by n and decrease in the order M3+ > M2+ > M+. 

Form Eq. (e) it is distinctly obvious that a series of several extractions would definitely prove to be more efficacious than one single extraction using the same total volume of solvent. 

From figure 27.1, the following steps may be adopted in order to determine the percentage of the analyte left in the sample after a single extraction ... 

The percentage of the analyte , left behind in the sample solvent after a single extraction is given by the intersection of the straight-edge with the right scale . 

In simple experiments, particulate silica-supported CSPs having various cin-chonan carbamate selectors immobilized to the surface were employed in an enantioselective liquid-solid batch extraction process for the enantioselective enrichment of the weak binding enantiomer of amino acid derivatives in the liquid phase (methanol-0.1M ammonium acetate buffer pH 6) and the stronger binding enantiomer in the solid phase [64]. For example, when a CSP with the 6>-9-(tcrt-butylcarbamoyl)-6 -neopentoxy-cinchonidine selector was employed at an about 10-fold molar excess as related to the DNB-Leu selectand which was dissolved as a racemate in the liquid phase specified earlier, an enantiomeric excess of 89% could be measured in the supernatant after a single extraction step (i.e., a single equilibration step). This corresponds to an enantioselectivity factor of 17.7 (a-value in HPLC amounted to 31.7). Such a batch extraction method could serve as enrichment technique in hybrid processes such as in combination with, for example, crystallization. In the presented study, it was however used for screening of the enantiomer separation power of a series of CSPs. 

B.4.2.2 Nonbenzodlazeptnes GG-NPD was used by Kivisto et al. (1999) to quantitate the nonbenzodiazepine anxiolytic buspirone and its major metabolite, l-(2-pyrimidinyl)-piperazine, using separate extraction methods and separate assays. The hmit of quantification in plasma for both compounds was 0.2 ng/mL, which makes this assay useful for pharmacokinetic studies of this compound (Kivisto et al., 1999). A rapid, simple method for analysis of buspirone in rat brain requiring a single extraction step followed by GC-NPD has also been described (Lai. et al., 1997). 

A single-stage extraction using the same total volume of solvent achieves only 92% extraction, and the extract concentration is only 0.23, vs. nearly 0.25 for the cross-flow extraction. The use of four cross-flow extraction stages is clearly preferable to a single extraction. Equally, of course, the use of more than four extraction stages, each with a proportionately smaller volume, would improve the performance. In the limit, one would seek a differential contacting process similar to the Soxhlet extractor employed for extraction from solid phases, but such a contactor has not found use in solvent extraction. 

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